This invention relates to optical signal processing and, more particularly, to a thin-film optical waveguide formed in an integrated optical structure.
Integrated optics is a recently developed technology in which thin-film techniques are applied to the fabrication of miniature compact optical devices. Due to their small size, rugged and reproducible construction, and low-cost potential, such devices are attractive candidates for inclusion in high-capacity optical communication systems.
For an overview of the field of integrated optics, see, for example, a "Survey of Integrated Optics" by S. E. Miller, IEEE Journal of Quantum Electronics, Vol. QE-8, No. 2, February 1972, pp. 199-205.
Techniques are known for producing a planar thin-film optical waveguiding layer on a supporting substrate material. In a planar waveguide of this type there is no optical beam spreading normal to the plane of the layer, but diffraction in the plane causes the beam to increase in width as it propagates along the film.
For some applications of practical interest, it is desired to limit the waveguiding portion of the layer to a longitudinal stripe. By so limiting the lateral extent of the optical beam, efficient modulators and other components needed to construct an optical communication system may be more easily realized. Moreover, the longitudinal stripe geometry facilitates the transfer of optical power from the thin-film structure to an associated optical fiber transmission line.
A longitudinal stripe waveguiding region may be formed in a planar film in a number of ways. For example, such a region may be formed by selectively irradiating the film through a mask. Or, by using standard photolithographic techniques, all but one or more guiding stripes of the film may be partially or completely etched away.
In some applications the known ways of forming a waveguiding stripe in a film are satisfactory. But in others they are not, due either to degradation of the properties of the stripe arising from the stripe-formation process or to the practical difficulty of forming the stripe in a particular materials system.